Direct-to-digital temperature sensor

ABSTRACT

A direct-to-digital temperature sensor is formed with a single switched-capacitor integrator, having a digital sequencer control.

PRIORITY CLAIM

[0001] This patent application has a priority date of Sep. 1, 1998 basedupon a provisional patent application, No. 60/098763, filed on thatdate.

FIELD OF THE INVENTION

[0002] The present invention relates generally to electronic devices.More particularly, the present invention relates to thermal managementby integrated circuits.

BACKGROUND OF THE INVENTION

[0003] Various processes require temperature monitoring for effectivecontrol. Such processes obviously include manufacturing processes, butcan also include transportation, security, maintenance, and other typesof processes during which monitoring the thermal characteristics ofdevices is necessary or advisable. Today, increasingly, manufacturingprocesses are automatically controlled; such processes generally requireelectronic temperature measurement. Further, microcontrollers andprocessors—common control means in automatic processes—require digital,as opposed to analog, temperature measurements.

[0004] Heretofore, the assignee of the present invention has developedthermal management products—primarily temperature sensors—that provide adirect-to-digital output. Because the sensors made by the assignee ofthe present invention provide a digital reading of temperature directly,any need for an A/D converter dedicated to temperature measurement iseliminated. Assignee's sensors also do not require inherently analog orexternal discrete components, such as thermistors, for proper operation.Because of the aforementioned characteristics, assignee's sensors caneasily be incorporated into integrated circuits.

[0005] Notwithstanding the work mentioned above, direct-to-digitaltemperature sensors still have a number of shortcomings. First andforemost, the costs of manufacturing are high, primarily because thedevices must be calibrated—generally trimmed—multiple times. Trimming,in turn, is essential to obtain even reasonable accuracy. Offsetvoltages and mismatched currents prevent even reasonable accuracy if notaddressed by trimming. In many cases, trimming is extremely expensivebecause a bath is required, which necessitates prepackaging withnon-volatile memory.

[0006] In view of all of the foregoing, there is a clear need for adirect-to-digital temperature sensor that has reduced manufacturingcosts, but high accuracy.

SUMMARY OF THE INVENTION

[0007] The present invention meets the aforementioned need by providinga direct-to-digital temperature sensor made up of a singleswitched-capacitor integrator, with a digital sequencer used to controlall required operations.

[0008] One advantage of the present invention is the fact that noprecise matching is required of any component pairs, because alloperations are reduced to a series of samples using the same capacitor.

[0009] Another advantage of the present invention is the degree to whichaccuracy can be easily obtained. This follows from the fact that theonly error source in embodiments of the present invention is the offsetvoltage of the integrator. Since there is just one offset, sophisticatedmethods can be used to reduce it.

[0010] Accordingly, an object of the present invention is to provide anextremely accurate direct-to-digital temperature sensor.

[0011] Another object of the present invention is to provide atemperature sensor that can be relatively inexpensively made.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The foregoing, as well as other features, advantages, and objectsof the present invention can be fully understood by reference to thefollowing detailed description of the invention, taken in conjunctionwith the accompanying drawings wherein:

[0013]FIG. 1 is a schematic diagram of a Vref generator;

[0014]FIG. 2 is a block diagram illustrating operation of ananalog-to-digital converter that can be used to generate digitaltemperature output in this technological area;

[0015]FIG. 3 is a schematic diagram of a temperature sensor according tothe teachings of the present invention; and

[0016]FIG. 4 is a graph showing relative accuracies of products madeaccording to the teachings of the present invention compared to productsmade according to prior art principles.

DETAILED DESCRIPTION

[0017] Referring now to the drawings and, in particular, to FIG. 1,there is shown a schematic diagram of a conventional Vref generator 10.Vref generator comprises a Vbe generator 12 and a Δ Vbe generator 14.Vbe is generated to have a negative temperature coefficient, i.e., TC<0.Δ Vbe, created in part by matched current sources 16, 18, isproportional to absolute temperature and, therefore, can be used torepresent temperature. Furthermore, as shown in FIG. 1, Δ Vbe has apositive temperature coefficient, i.e., TC>0. Generator 10 produces aVref obtained from Vbe+B* Δ Vbe, with B chosen so that Vref has a zerotemperature coefficient.

[0018] Conventional temperature sensors must also generate voltages Vin.Vin, representing temperature, can also conventionally be obtained fromA*Δ Vbe.

[0019] Referring now to FIG. 2, there is shown a conventionalanalog-to-digital converter (AD/converter) 20. Converter 20 acceptsinputs Vin and Vref, generated as discussed above, and produces anoutput Nout by Quantizing Nmax*Vin/Vref. Based upon the value ofVin/Vref, a value Nout is produced, varying from 0 to Nmax, as follows:Vin/Vref Nout 0 0 0.1 0.1 Nmax 0.2 0.2 Nmax . . . . . . . . 1 Nmax

[0020] As is known to those skilled in the art, in many AD/convertermethods a critical operation is (Vx+/−Vref). Vx can be an intermediateinternal voltage or, in the case of a Delta-Sigma Converter, it can beVin itself.

[0021] To perform the functions mentioned above, several analogoperations are needed; most notably, two multiplications [(A * Δ Vbe)and (B * Δ Vbe)], to obtain Vin and an intermediate value to be summedwith Vbe to obtain Vref, respectively; and two additions/subtractions[(Vbe+B* Δ Vbe) and (Vx+/−Vref)], to obtain Vref and the intermediateinternal voltage discussed above, respectively.

[0022] Most implementations of these analog operations suffer fromamplifier input offset voltage and/or offset current, and limitedmatching capacity of electrical components such as resistors,capacitors, transistors, and the like. Typically, multiple amplifiersand multiple matched pairs are used in implementing the entire system.Each amplifier contributes its own offset and each matched paircontributes its matching error. Since each error has its owncharacteristics with respect to operating temperature and voltage,mechanical stress, and shift in time, the total error cannot be easilytrimmed out using one or more calibrations. Referring now to FIG. 3,there is shown a temperature sensor according to the teachings of thepresent invention. This sensor effectively reduces all of theaforementioned analog operations into a single switched-capacitorintegrator using a digital sequencer to control all required operations.

[0023] There are only three inputs to the entire system: Vbe0, Vbe1, andVbe2. These inputs, as shown in FIG. 3, can be directly output (in thecase of Vbe 0), output from a first transistor 22 (in the case of Vbe1),or output from a second transistor 24 (in the case of Vbe2). +Δ Vbe canbe generated by Vb_(e1)-Vb_(e2). −Δ Vbe can be generated by Vb_(e2)−Vb_(e1). Similarly, −Vbe can be generated by Vbe2−Vbe0 and +Vbe can begenerated by Vbe0-Vbe2. Thus, by effective control of transistors 22 and24, the two main values used to generate Vref, discussed above withrespect to FIG. 1, can be generated.

[0024] The aforementioned subtractions can be performed by sequencer 26,closing switches 28, 30, 32 appropriately. As is well known to thoseskilled in the art, in switched-capacitor circuits, an addition orsubtraction differs only in the switching sequence which is handled bythe digital sequencer 26. In FIG. 3, capacitor Cin 34 is the switchedcapacitor on which the voltages Vbe and Δ Vbe can be captured.

[0025] Since Vin can be written as (A*Δ Vbe) and −Vref is −(Vbe+B* ΔVbe), a complex operation such as (Vin-Vref) can be performed asfollows.

[0026] 1) +(ΔVbe) is sampled A times;

[0027] 2) −(ΔVbe) is sampled B times; and

[0028] 3) −(Vbe) is sampled one time.

[0029] Such successive sampling can be readily accomplished by controlof sequencer 26, in a manner well known to those skilled in the art.

[0030] The above described technique offers a number of advantages.First of all, because all operations are reduced to a series of samplesusing the same capacitor Cin 34, no precise matching is required of anycomponent pairs. Secondly, the only error source is the offset voltageof the integrator 36. Since there is just one offset, sophisticatedmethods can be used to reduce it. For example, one such method isCorrelated-Doubled Sampling (CDS), a holding method in which a separatecapacitor is used to sample the offset. Another possible dynamic offsetcancellation technique is a digital one that is unique to one-shot typeof conversions such as temperature measurement. Since the entiremeasurement is completed before another one is started, the effect ofoffset, Vos, can be reversed halfway through the conversion. In equationform:

Nout=½*(N1-N2)

N1=Nmax * (Vin+Vos)Vref

N2=Nmax * (−Vin+Vos)/Vref

Nout=½* [(Nmax * (Vin+Vos/Vref]−[Nmax * (−Vin+Vos)/Vref)]

[0031] Note: (Nmax * (Vin+Vos)/Vref)=N1, and (Nmax * (−Vin+Vos)/Vref)=N2

Nout=½* Nmax * (2*Vin+Vos−Vos)/Vref

Nout=Nmax * Vin/Vref

[0032] Such reversal effectively “cancels out” the offset error. Asmentioned previously, Vin or −Vin is obtained by simply changing thesampling sequence.

[0033] In generation of Δ Vbe in embodiments of the present invention,two identical copies of a biasing current are used to bias two bipolartransistors (shown) or diodes (not shown) with different effectiveareas. Mismatch between currents can readily be eliminated. There aretwo phases in each cycle: a sampling phase and an integration phase. Thetwo currents are swapped between the two phases in the circuit thatgenerates Vbe1 and Vbe2. The effective biasing current at the end ofeach cycle is the mean of the two currents.

[0034] Based upon the foregoing, those skilled in the art should nowrecognize and appreciate that the present invention provides atemperature sensor that reduces all basic analog operations into asingle switched capacitor integrator, using a digital sequencer tocontrol all required operations. Thus, the present invention provides atemperature sensor that is relatively inexpensive to manufacture, butalso very accurate. With regard to the latter aspect, the only errorsource in the invention—offset error of the integrator—is easily subjectto reduction methods.

[0035] Referring now to FIG. 4, there is a graph that should give thoseskilled in the art an appreciation of the relatively small accuracydrift of a product made according to the teachings of the presentinvention (the DS1721), compared to a prior art direct-to-digitaltemperature sensor (the DS1621). From FIG. 4, it should be clear howmuch more accurate products made according to the teachings of thepresent invention are compared to products made according to prior artprinciples. This increased accuracy is particularly astoundingrecognizing that products made according to the teachings of the presentinvention require much less trimming, and thus are much cheaper to make,than products made according to prior art principles.

[0036] Attached as Appendix A is a preliminary data sheet providingdetails about an embodiment of the present invention subject todevelopment efforts by the assignee of the present invention. Appendix Ais attached as an example only, of a possible embodiment of the presentinvention.

[0037] Obviously, numerous modifications and variations are possible inview of the teachings above. Accordingly, within the scope of theappended claims, the present invention may be practiced otherwise thanas is specifically described above.

What is claimed is:
 1. A temperature sensor comprising: aswitched-capacitor integrator; and a digital sequencer control.
 2. Thetemperature sensor of claim 1 , further comprising means for cancelingintegrator offset voltage.
 3. The temperature sensor of claim 2 ,wherein said means for canceling comprises a separate capacitor used tosample integrator offset.
 4. The temperature sensor of claim 3 , whereinsaid separate capacitor allows implementation of correlated—doubledsampling.
 5. The temperature sensor of claim 2 , wherein said means forcanceling comprises means for reversing integrator offset halfwaythrough conversions.
 6. The temperature sensor of claim 1 , furthercomprising means for eliminating mismatch between two different biasingcurrents.
 7. The temperature sensor of claim 6 , wherein said means foreliminating comprises means for swapping sampling and integration phasesin Vbe1 and Vbe2 generation circuits.
 8. The temperature sensor of claim6 , wherein said biasing currents are used to bias two bipolartransistors with different effective areas.
 9. The temperature sensor ofclaim 6 , wherein said biasing currents are used to bias diodes withdifferent effective areas.
 10. The temperature sensor of claim 1 ,wherein there are only three inputs to the system.
 11. A systememploying thermal management components comprising: a switched-capacitorintegrator; and a digital sequencer control for said switched capacitorintegrator.
 12. The system of claim 11 , further comprising means forcanceling offset errors in said thermal management components.
 13. Thesystem of claim 11 , further comprising means for eliminating currentmismatches in said thermal management components.
 14. A method ofdetecting temperature comprising the steps of: sampling voltages using aswitched-capacitor integration; and controlling sampling with a digitalsequencer.
 15. The method of claim 14 , further comprising the steps ofcanceling integrator offset error.
 16. The method of claim 14 , furthercomprising the step of eliminating biasing current mismatches.